The bio-stability of ultraviolet disinfected reused water from a tertiary biological wastewater treatment plant was investigated, using laboratory-scale simulated static water pipelines. UV disinfection was found not to be able to maintain its disinfection effect over a few days, such that the total bacterial count increased up to the same level as that for non-disinfected samples in only 5 days. Higher temperature enhanced the increase of bacterial counts in UV-treated samples, while at the same time decreasing that for non-disinfected samples. The biofilm concentrations of bacteria in the effluent for UV or chlorine treated samples were different, but reached similar values after 18 days. UV disinfection led to no significant differences in the microorganism community, as compared to other disinfection treatments, Such as chlorine.